PROJECT SUMMARY CD25 was previously identified as ?-chain of the heterotrimeric IL2 receptor (IL2R). High expression levels of CD25 was previously attributed to T-cell activation or transcriptional activation by Foxp3 in regulatory T-cells (Tregs). Our preliminary experiments revealed that CD25 is not only an IL2R chain, but in fact binds the CD3 signal chains of the T-cell receptor (TCR) for feedback control of TCR signaling strength. Reflecting its central role in normal T-cell development, the TCR and its downstream signaling pathway is a target of oncogenic transformation in the majority of peripheral T-cell lymphomas (PTCLs). Oncogenic TCR-mimics promote survival and proliferation even in the absence of a functional TCR. Oncogenic TCR-mimics include activating lesions of the TCR-signaling chain CD3?, proximal tyrosine kinases (PTKs; FYN, LCK, ZAP70) and ITK-SYK, NPM1-ALK fusions. In collaboration with other investigators of this P01, our group recently discovered that lymphoid malignancies are uniquely dependent on feedback regulation of PTKs (Chen et al., Nature 2015), PI3K (Shojaee et al., Nature Med 2016; Chan et al., Nature 2017) and ERK (Shojaee et al., Cancer Cell 2015; Xiao et al., Cell 2018). Here we validate the concept that in multiple PTCL subtypes, CD25 orchestrates feedback control of all three pathways (PTKs, PI3K and ERK). The central goal of this project is to deliver a comprehensive strategy to target CD25-mediated feedback control of oncogenic TCR signaling to overcome drug resistance in refractory PTCL. The following three Aims will (1) elucidate the mechanism of CD25-mediated feedback control of oncogenic TCR-signaling in PTCL subtypes, (2) provide a rationale for combining PI3K inhibitors with CD25 antibody-drug conjugates (ADC) and (3) evaluate efficacy of newly developed CD25 chimeric antigen receptors (CARs) engineered in T- and NK-cells and their effects on endogenous anti-tumor immune responses. Aim 1: Mechanisms of CD25-mediated feedback control of oncogenic TCR-signaling in PTCL. Aim 2: CD25-endocytosis and endosomal recycling as dynamic feedback control of oncogenic TCR-signaling Aim 3: Preclinical development and validation of CD25 CAR-T and CAR-NK cells.